(MONEY Magazine) – Your patience melts along with your Haagen-Dazs as the shopper ahead of you in the supermarket checkout line extracts a wad of cents-off coupons from his pocket. You scan the tabloid headlines (WEDDING EVE SHOCK FOR TV STAR, FACELIFTS FOR PETS) when he hauls out his checkbook. You sigh as the clerk sidles off to the manager to get the check approved. Bide awhile. Soon such aggravating delays will be a thing of the past as retailers across the country embrace a substitute for checks and even money -- the point-of-sale cash card, more formally known as the debit card. A cash card looks like a credit card, but instead of charging purchases with your plastic, you pay for them immediately. (See the illustration below for an explanation of how cash cards work.) Transactions take about six seconds, giving the customer in line behind you barely enough time to scan those headlines. About 46,000 supermarkets, gasoline stations, fast-food restaurants, department stores and other retail establishments are now ringing up cash-card sales, compared with a pioneering 100 six years ago. All told, some 100 million such transactions are expected to take place this year, nearly double the number in 1987. Most cash-card systems are sponsored by banks that limit use of their cards to participating stores in a single state or a handful of adjacent states. A few cash cards are acceptable in every state; for instance, clients of some large brokerage houses get Visa cards that function like cash cards. But a true national system awaits a major marketing campaign by the two big bank- card organizations, Visa U.S.A. and MasterCard International, for their joint Entree cash card. Banks have issued about 1.5 million Entree cards, but no store is yet equipped with terminals that accept them. In July, the attorneys general of New York, California and 10 other states filed an antitrust suit charging that Visa and MasterCard have conspired to stifle Entree's use to preserve the higher fees that member banks get on credit cards -- typically $20 a year for the card plus 18% annual interest on unpaid balances. The credit-card companies deny the accusation, blaming the banks for not persuading merchants to buy or lease the terminals, which can cost as much as $1,200. For consumers, the major appeal of cash cards is, of course, convenience. You don't have to carry much cash or worry about stores refusing to accept your checks. The disadvantage is the loss of what bankers call float, the time between a purchase and the deduction of your payment from your bank account. Shoppers who pay by check enjoy at least a couple of days' float. The grace period for credit-card users who pay their monthly bills in full can be as long as 60 days.

Not all cash cards take away the float. For example, cards issued by Mobil Oil and Vons, a Southern California supermarket chain, give two days' grace. Says Virginia Miller, vice president and treasurer of Vons: ''Retaining a float helps families whose weekly paycheck has not cleared the bank by Saturday or Sunday, when most people shop.''

Cash cards are, in fact, more liquid than cash. If you have shopaholic tendencies when you use a credit card, watch out. A cash card gives you instant access to every dollar in your checking account plus all of your overdraft credit. Other pros and cons to consider: -- Fees. Most banks and retailers have so far been picking up the costs. The exceptions include Mobil, which charges a $12 annual membership fee, and a handful of banks that tack on a few cents for each point-of-sale transaction. -- Cash discounts. At gas stations, cash-card purchases qualify for cash discounts, usually 4 cents off a gallon. Vons supermarkets are experimenting with an electronic equivalent to cents-off coupons.

-- Liability. The risk of loss if a thief raids your account may be far less than you think. The law makes you responsible for up to $50 in charges if you notify the bank within two business days after you learn of an unauthorized withdrawal. For the next 60 days, if you still haven't called the bank, your liability rises to $500. Wait any longer and your potential loss is limited only by your bank balance plus any credit line in your account. But Barry Schreiber, a professor of criminal justice at St. Cloud State University in Minnesota, who has surveyed bank policies, says: ''Most banks reimburse people for unauthorized charges because they want to establish that your risk of loss from bank account raiding is quite low.'' To play it safe, take these precautions: Don't write your personal identification number (PIN) on your card or keep it with your card. That seems obvious. Yet a couple of years ago, a bank discovered that 25% of the cardholders in its automated teller machine system wrote their PINs on the signature strip. When you report a card stolen, the bank will cancel it, blocking further transactions on your PIN until a new card and a new number are issued. That can take seven days. Choose a PIN you can easily remember. Cash-card issuers recognize that life is becoming a maze of numbers too long and too numerous to recall. So more and more of them let you pick your own number. But don't select one that is on display, like your house number or your automobile license plate. Don't reveal your PIN even to friends and relatives. Surprisingly, bankers track most challenged charges not to thieves but to relatives and friends impulsively using a card without the holder's permission.